The viability of butanol as a biofuel option in western Colorado

Andrew Brandess, Can Erbil, Catherine Keske* (cerbil@brandeis.edu)

Research from Colorado State University’s departments of Agricultural and Resource Economics and Soil and Crop Science has started targeting marginal lands in western Colorado for biofuel growth, specifically bio-butanol. The limited water resources in the western part of the state have led to unique agronomical needs, specifically the growth of crops that are not resource intensive. Colorado State University Extension has ongoing field trials with switchgrass, alfalfa, and other perennial grasses, both native to Colorado and introduced from out of state. These grasses are intended to be used as feedstock for butanol fuel, an alternative to petroleum based gasoline.

 

Biofuels have received many critiques recently due to the “food for fuel” debate, an ongoing conversation that has been primarily led by the opposition to the ethanol industry. While ethanol in the United States is corn based, and does result in the diversion of food to instead power vehicles, butanol grown in Colorado is able to circumnavigate the debate. All the lands proposed to grow butanol feedstocks are considered marginal lands which otherwise would remain vacant. These lands are marginal due primarily to the lack of reliable irrigation that is greatly needed in the arid western half of Colorado.

Butanol has been suggested as an alternative to ethanol due to better fuel qualities (Szulczyk, 2010). Willke and Vorlop (2004) suggest that butanol is an attractive alternative because of the high intrinsic calorific value compared with ethanol, the low vapor pressure, and the tendency for butanol to not mix with water (Willke and Vorlop, 2004). Butanol is also applicable to the current pipeline infrastructure in the United States (Szulczyk, 2010). Another advantageous aspect of butanol is the reported positive “plow to tire” equation, suggesting that more energy is released from the combustion of butanol than is needed to create the liquid transportation fuel (Ramey and Yang, 2004). With regards to ethanol, this “plow to tire” energy equation is often considered negative.

Butanol is produced using the Acetone-Butanol-Ethanol (ABE) fermentation process (Ramey and Yang, 2004). ABE fermentation is a process in which the carbohydrate substrate is converted to a mixture of solvents, acetone, butanol, and ethanol. It is converted in a 3:6:1 ratio of each chemical, respectively (Karakashev and Angelidaki 2010). Raw feedstock is fermented using the Clostridium acetobutylicum bacteria, one of the oldest known industrial fermentations (Ramey and Yang, 2004).

Butanol is a straight chain of four carbon atoms, with oxygen and hydrogen atoms attached to the end of the molecule. This atom alignment, known as a hydrocarbon, is similar to the hydrocarbon chain that makes up conventional gasoline and differs from the atom structures that create ethanol. As a result, butanol can be blended into gasoline in any ratio as an engine does not need to be modified to run on butanol. This is not the case for ethanol, which has limits to the amount that can be blended into an unmodified engine (Szulczyk 2010). While the maximum concentration for ethanol in a standard automobile engine is currently 15%, an engine can run on 100% butanol without modifications (Ramey and Yang, 2004).

Another positive property for butanol is the amount of oxygen in the fuel. Szulczyk (2010) and many others report that more oxygen in the fuel leads to a more complete combustion, which in turn reduces carbon dioxide emissions. Gasoline has almost no oxygen content, while butanol is comprised of nearly 22% oxygen (Brekke, 2007; Davis and Diegel, 2006; Gallagher et. al., 2003). To combat the release of carbon monoxide from incomplete combustion, the Environmental Protection Agency had mandated the use of methyl tertiary-butyl ester (MTBE) as an additive to use in gasoline. Once MTBE was linked to ground water contamination through its high levels of solubility, it was replaced with ethanol as an additive meant to oxygenate fuel (Szulczyk, 2010). The higher oxygen percentage than gasoline suggests that butanol does not require any additives and is cleaner burning that gasoline. Butanol also has a significantly lower Reid vapor pressure than gasoline, at .023 bars compared with .48 to 1.034 for gasoline (Ramey and Yang, 2004). This suggests that butanol does not vaporize as readily as gasoline. Szulczyk (2010) reports that higher vaporization rates lead to higher pollution levels, as ultraviolet radiation from the sun converts the volatile organic compounds released from vaporization into harmful ground ozone pollution. While a low Reid vapor pressure can cause difficulties for starting an engine in very cold weather, a lower Reid vapor pressure is advantageous under most operating conditions due to reduced emissions.

Butanol also has an octane number of 87, which is comparable to that of gasoline (ACFA, 2008). Octane is a measure of how much pressure and temperature are needed to ignite the fuel mixture (Szulczyk, 2010). Premature ignition of the fuel can create engine knocking, which causes stress and can damage the engine (ACFA, 2008). This is an important fuel attribute for butanol which allows for it to be directly substituted in engines for petroleum based gasoline.

Another advantage of butanol is the ability to transport it in the current commercial pipeline infrastructure. Butanol is less hygroscopic than ethanol, resulting in a less corrosive fuel that will not damage existing pipelines (Karakashev and Angelidaki 2010). This is not the case for ethanol.  Ethanol-gasoline blends have also been reported to separate in water, which is not noted in butanol and butanol-gasoline blends. This suggests that butanol can be stored in the same tanks as petroleum based gasoline, as opposed to the separate tanks that are required for ethanol (Szulczyk, 2010). Since butanol is not miscible with water, there is considerably less danger of contamination with ground water as there is with ethanol, a highly soluble solution (Szulczyk, 2010).

A minor drawback of butanol is that is has lower heating value of 27.8 MJ/liter, compared with gasoline’s lower heating value of 31.2 MJ/liter. While this is a moderate reduction, it should be noted that the lower heating value of ethanol is 21.1 MJ/liter (Szulczyk, 2010). The lower heating value is the most commonly used measure of fuel energy content (Gerpen et al., 2004). It is the measure of the amount of heat energy released by the fuel, excluding energy used to vaporize water. The lower heating value is directly linked to mileage that a vehicle can travel from a gallon of fuel. While it is a drawback that a gallon of butanol has only about 86% of the energy content of a gallon of gasoline, it is a significant improvement over ethanol, which only has about 65% of the intrinsic energy (Szulczyk, 2010). Another drawback that has been noted is the cost for the ABE fermentation process. The process is not efficient as it can be as distillation is needed to separate butanol from acetone, which results in added expense and less butanol output. Research has been focusing on the enzymes used to breakdown the feedstock substrates in order to more efficiently separate butanol molecules.

The research at Colorado State University is primarily focused on the cost of growing the butanol feedstock. A budget generator has been created that allows the user to manipulate the cost and quantity of nearly every input used on-farm, from the cost and quantity of nitrogen to the interest rate that farmers borrow at from lenders for operating capital. Using a complete on-farm enterprise budget, users can compare four crop scenarios (switchgrass, tall fescue, an introduced crop mixture that includes alfalfa, and a native to Colorado wheatgrass mixture) to determine the breakeven cost, per ton, of growing the butanol feedstock on their marginal land.

Over the coming months, a complete “plow to tire” budget will be created to determine the economic feasibility of replacing the gasoline needs of the entire fleet of city vehicles in the city of Rifle, Colorado, with butanol. While Rifle is a smaller city with a population of only about 7,000 people, it offers an excellent case study to determine the economic feasibility of growing butanol in a closed rural economy. The study will not only include the cost of growing the butanol feedstock, which has already been finished, but also map the cost of transporting the feedstock to a processing center, the cost of building a butanol processing facility, and the savings from displaced gasoline, both in pure accounting terms as well as from the standpoint of money saved from reduced emissions.

Butanol is a biofuel that will likely have success in western Colorado and other arid western states. While it will not likely be the best biofuel option elsewhere, it does offer an opportunity for significant western contribution to the Renewable Fuel Standards. In the coming months, more research will offer better economic insight into the future of butanol, and supplement the claims that the feedstock is a better alternative to both ethanol and gasoline.

References

Asian Clean Fuels Associations. “Octane – What’s behind the Number.” ACFA News October 2008, 6(7), 1-3. Available at http://www.acfa.org.sg/pdf/acfa1008.pdf

Brekke K. “Butanol-An Energy Alternative?” Ethanol Today. March 2007

Davis SC, Diegel SW. Transportation Energy Data Book: Edition 25, Oakridge, TN: Center for Transportation Analysis, Oak Ridge National Laboratory, Report ORNL-6974.

Gallagher PW, Shapouri H, Price J, Schamel G, Brubaker H. “Some Long-run Effects of Growing Markets and Renewable Fuel Standards on Additives Markets and the US Ethanol Industry.” Journal of Policy Modeling 2003, 25, 585-608.

Gerpen, JV, Shanks B, Pruszko R, Clements D, Knothe G. Biodiesel Analytical Methods: August 2002-January 2004. Golden, CO: National Renewable Energy Laboratory, Report/NREL/SR-510-36240, July 2004.

Karakashev, Dimitar, and Irini Angelidaki. 2010. Emerging Biological Technologies: Biofuels and Biochemicals. In Solid Waste Technology & Management: John Wiley & Sons, Ltd.

Ramey, David, and Shang-Tian Yang. 2004. Production of Butric Acid and Butanol from Biomass. Final Report. U.S. Department of Energy, Morgantown WV.

Szulczyk, Kenneth R.  2010. Which is a Better Transportation Fuel- Butanol or Ethanol? International Journal of Energy and Environment 1 (3):501-512.

Willke, Th, and K. D. Vorlop. 2004. Industrial bioconversion of renewable resources as an alternative to conventional chemistry. Applied Microbiology and Biotechnology 66 (2):131-142.

 

* USDA, Brandeis University and Colorado State University, respectively.

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